Target creature gets +X/+X until end of turn, where X is your devotion to green. (Eachin the mana costs of permanents you control counts toward your devotion to green.)
9/15/2013 Numeric mana symbols (, , and so on) in mana costs of permanents you control don’t count toward your devotion to any color.
9/15/2013 Mana symbols in the text boxes of permanents you control don’t count toward your devotion to any color.
9/15/2013 Hybrid mana symbols, monocolored hybrid mana symbols, and Phyrexian mana symbols do count toward your devotion to their color(s).
9/15/2013 If an activated ability or triggered ability has an effect that depends on your devotion to a color, you count the number of mana symbols of that color among the mana costs of permanents you control as the ability resolves. The permanent with that ability will be counted if it’s still on the battlefield at that time.
2/1/2014 The value of X is calculated as Aspect of Hydra resolves. The bonus won’t change later in the turn, even if your devotion to green does.